Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly the same habits of
life as some of their Australian relatives, having feet constructed on the
ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion: "We may call this conformity to type, without
getting much nearer to an explanation of the phenomenon;" and he then adds
"but is it not powerfully suggestive of true relationship, of inheritance
from a common ancestor?"
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the
arm and forearm, or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
a beetle? Yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae.
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